You can place seedlings precisely
where you want them instead on the somewhat hit and miss
method of sowing directly into a garden bed.

Turnover

While a crop in the ground is
maturing you can be already growing the next crop in as
seedlings. Once the crop in the ground is harvested and
manured you can immediately plant your seedlings that have
already had four weeks growth, thus reducing the turnover
time (the time from planting through to harvest and then
making the bed ready for the next planting) by around four
weeks.